One method for preheating glass batch involves feeding cold particulate glass batch raw materials into one end of a rotating heat-transfer drum, and feeding hot media of larger particle size than the batch particles into the other end of the heat transfer drum. The glass batch moves in direct and immediate physical contact with the heated media, with the batch flowing from the cold end to the hot end of the drum and the media flowing from the hot end to the cold end of the drum. The heated particulate batch is removed from the hot end of the drum, and the cooled media is removed from the cold end of the drum. Preferably, the heat transfer media is of a durable material and can be comprised of glass batch agglomerates, glass, ceramic material, steel, stainless steel, aluminum, or gravel. The media can be spherical in shape, and a useable example of such media is spherical ceramic balls. The media can be heated with an external burner or preferably heated by direct contact with exhaust gases from a glass melting furnace.
A problem with such a preheating apparatus and method is that it is not always desirable to direct the cold batch into the batch preheating apparatus. Therefore, it is necessary to provide a batch bypass conduit so the batch can be transferred directly from the batch supply into the glass melting furnace. It has been found, however, that the time required to effect a simple switching of directing the batch either into the batch preheater or into the batch bypass line can be a long period of time, such as up to ten hours or more. This is because the batch preheating method has the undesirable effect of driving off one of the glass batch constituents, thereby changing the chemical composition of the heated glass batch being directed from the batch preheater into the glass melting furnace. Specifically, it has been found that the borax constituent of the particulate glass batch is intumesced in the batch preheater, and the intumesced borax is removed with other vented gases during the preheating process. In order to accommodate the loss of the borax in past attempts to preheat batch, the borax component of the cold batch had to be increased. Thus, a separate batch supply, having an increased borax component, was needed to supply the glass melting furnace when the batch preheater was to be in use. This made it difficult and time consuming to switch from an operation using the batch preheater to an operation in which the batch preheater was not employed. There has now been developed a method and apparatus for preheating particulate glass batch in which the heated glass batch contains substantially the same chemical constituents as the cold particulate glass batch.